This invention relates to apparatus for blending solid particulate material such as plastic pellets and in particular to a fines separations system for such a pellet blender.
Prior to the present invention, various apparatus have been utilized for blending or mixing pelletized materials such as plastic pellets. This apparatus may consist of generally vertically oriented vessels adapted to be at least partially filled with particulate material. In order to blend the material, the material is withdrawn from various levels in the vessel to a common point through vertical downcomers. In some instances, the material is recirculated back into the top of vessel to achieve the blending or mixing while in other cases the material is withdrawn from the various levels in vessels for direct discharge from the vessel to a use point.
In some instances such as illustrated in U. S. Pat. No. 4,569,596 issued Feb. 11, 1986, and in U. S. Pat. application, Ser. No. 06/848,005 filed Apr. 3, 1986, there is a central lift column with gaseous fluid under pressure supplied to the lower inlet of the lift column to entrain material and lift it up through the column to its outlet where it cascades in a geyser-like manner into the top of the vessel to thereby blend the material within the vessel.
Material to be blended is often supplied to the vessel by pneumatic conveying means. Typically, this is done by supplying the material to the top of the vessel, but in some instances such as illustrated in the aforesaid U. S. Pat. No. 4,569,596, the material may be supplied to the bottom of the vessel for direct passage up through the lift column. This system utilizes the conveying air as the blending energy.
It is generally known that a quantity of plastic pellets may contain a fine fraction of material which is smaller than the desired pellets. For example, a typical desired pellet may have a length by diameter of 1/8 by 1/8 and anything smaller than the specified size would be considered a fine fraction and not a usable product.
In some instances some pellets may have a particle size within the desired range but during the production process there is an occurance which results in a particle that is less dense or lighter in weight than the desired product. Having a quantity of less dense pellets may be the result of operating the extruder at a high production rate.
As used in this specification, "fine fraction" encompasses both the less dense pellets or particles and the smaller particle size. It would be desirable to eliminate this fine fraction.
It is known prior to the present invention that if gaseous fluid is disbursed through a quantity of pelletized material contained within a vessel, the gaseous fluid will entrain the fine fraction and this fine fraction can be discharged from the vessel through the vent of the vessel to a dust collection apparatus. However, in most instances, a high pressure drop is encountered in the cleaning process and a satisfactory result may not be obtained. The fines cleaning process is broadly described in the aforementioned U. S. Pat. No. 4,569,596.
It would be desirable to utilize the energy which is used for blending the particulate material or pellets to serve as a means for separating the fine material from the quantity of desired sized pellets. It would also be desirable if the energy utilized to supply the particulate material to the blender or vessel could also be utilized in cleaning the pellets by removing the fine fraction of material. If the pellets can be cleaned by removing the fine fraction during the supply process, then there will not be a build-up of fine material within the vessel.
During the course of manufacture and use of pelletized material it is sometimes desirable to manufacture one size pellets and then at some time manufacture a different size pellet. Since the same blender/storage vessel may be utilized for different sized pellets, it would be desirable to have an apparatus which allows adjustment of the split between the fine fraction to be removed and the coarse fraction to be retained.